Sasuke getting healed by karin
So even without Ka, sasuke might of died of his wounds.
Danzo's Eye started working Here
Yet he chose to Grab karin instead of instantly using Ka.
Read what Tobi Says: "He can't decide to use it as a Genjutsu (KA), or Throw it away like izanagi. ALso Danzo states "i should use shisui's genjutsu on Madara".
Aswell as stalling, he gave sasuke a little speach while he was holding Karin, he could of easily used KA if he really wanted to.